A. Field
This invention relates to apparatuses for examining magnetic properties of objects, in particular sheet material, such as bank notes. The apparatuses comprise a magneto-optic layer whose optical properties are influenceable by the magnetic properties of the object, a light source for producing light that is coupled into the magneto-optic layer, and a detector for detecting light that is transmitted and/or reflected by the magneto-optic layer.
B. Related Art
To guarantee high forgery-proofness, bank notes are provided with magnetic features, among other things. During automated bank note testing in bank note processing machines, bank notes are therefore also examined for their magnetic properties to distinguish counterfeits or suspected counterfeits from authentic bank notes.
Examination of the magnetic properties of bank notes is usually effected in this connection using inductive measuring heads, Hall elements or magnetoresistive elements, such as field plates or thin permalloy layers.
In addition, it is known to examine the magnetic properties of bank notes using magneto-optic layers. A suitable apparatus is known for example from German laid-open print DE 197 18 122 A1. A magneto-optic reflector layer with a high magnetic Kerr effect is illuminated with polarized light and the reflected light detected after passing through a polarizing filter. If a bank note under examination is brought close behind the reflector layer, the magnetic leakage flux of the magnetic areas of the bank note influences the optical behavior of the reflector layer, thereby changing the polarization direction of the detected light. From the measured change of polarization one can then infer the magnetic properties of the sheet material.
Compared to other measuring methods and assemblies, e.g. inductive measuring heads, the use of magneto-optic layers has the advantage that they allow high spatial resolution and the measurement of magnetic flux is independent of the speed of the bank note relative to the measuring system.
Examination of the magnetic properties of bank notes by machine involves in particular the problem that very small magnetic flux densities must be detected to be able to guarantee a sufficiently precise and reliable check of authenticity. This is because, firstly, the leakage flux caused by the individual magnetic areas of the bank notes is very small and, secondly, the typical distances between bank note and magneto-optic layer cannot be reduced at will due to the high transport speed required in bank note processing machines, as this would otherwise lead to elevated wear of the bank notes to be checked and individual sensor components and in addition result in an elevated risk of jams.
The problem to be resolved by the present invention is to provide apparatuses and a method that allow more precise and reliable examination of magnetic properties of sheet material.